“s” keyboard key on my Dell Inspiron 6400 does not work

I have a problem that started two day ago. My “s” key does not work on my Dell Inspiron 6400 laptop (Window XP Professional SP3 ). In my on-screen keyboard the “s” key does work (I am typing this message through “copy & paste” the “s” from an old Word text file.
DOES ANYONE HAS A VALID SOLUTION TO THIS PROBLEM ?
Thank you !